Measurement of the Cosmic Ray Spectrum above 4 $\times$ 10$^{18}$ eV Using Inclined Events Detected with the Pierre Auger Observatory

Abstract

A measurement of the cosmic-ray spectrum for energies exceeding 4×10(18) eV is presented, which is based on the analysis of showers with zenith angles greater than 60° detected with the Pierre Auger Observatory between 1 January 2004 and 31 December 2013. The measured spectrum confirms a flux suppression at the highest energies. Above 5.3×10(18) eV, the ``ankle'', the flux can be described by a power law E(−)(γ) with index γ=2.70 ± 0.02 (stat) ± 0.1 (sys) followed by a smooth suppression region. For the energy (E(s)) at which the spectral flux has fallen to one-half of its extrapolated value in the absence of suppression, we find E(s)=(5.12±0.25 (stat)(+1.0)(−)(1.2) (sys))×10(19) eV.